Fiber reinforced plastics are a broad class of thermoplastic and thermosetting resins characterized by their strength, corrosion resistance, good dielectric properties and moldability. Sheet molding compounds (SMC) are a subclass of fiber reinforced plastics which are widely used in the manufacture of reinforced plastic products such as automobile parts, particularly in the area of automotive exterior skin applications to reduce the weight of components such as grill-opening panels, fenders, decklids, hoods, and door skins (Kirk-Othmer, Encyclopedia of Chemical Technology, Third Ed., volume 18, Wiley Interscience, p. 590). SMCs also have applications in power-tool housings, business machine components, shower stalls, and so forth.
Typical SMC formulations generally contain an unsaturated polyester, styrene, a low profile additive, thickening agent and various additives and fillers. When used in a weight reducing application, SMC parts are molded as two rigid pieces, a smooth surface plastic skin and a reinforcing base plastic member. The skin is then bonded to the reinforcing member with a suitable adhesive and baked to cure to form the finished part. Until comparatively recently, urethane based adhesives were the adhesives of choice in the automotive industry to bond SMC pieces when forming SMC parts.
Much attention is now being focused, however, on electrocoating paint processes in automotive assembly line operations which require process temperatures generally in excess of the decomposition temperature of most urethane adhesives. In an electrocoating process, an electrical charge is applied to an assembly unit to be primed. The charged unit is coated with pigment by submerging in a bath containing pigment particles which are oppositely charged. The assembly is then removed from the pigment bath and cured in a baking oven for a period of about 10 minutes to about 60 minutes at a temperature typically ranging between about 350.degree. and about 450.degree. F. Electrocoating is very desirable as an effective means of sealing undersurfaces and other areas not reached by conventional methods of paint application.
Urethane based bonding systems, however, typically decompose at temperatures ranging between about 250.degree. F. and about 350.degree. F., and can thus show extensive adhesive failure when exposed to electrocoating process temperatures used to make painted SMC parts. Accordingly, researchers have developed bonding systems capable of withstanding the higher temperatures of these processes. Epoxy based adhesive systems in particular have been of considerable interest. Fusor.RTM.310, available from Lord Corporation, Chemical Products Group, is an example of a commercially available temperature resistant epoxy bonding system which is marketed for use with SMC substrates and which presently appears to be enjoying increased use in the automotive industry.
Although epoxy based systems such as Fusor.RTM.310 are able to withstand high electrocoating process temperatures without decomposing, until this invention substrates bonded with these systems required surface sanding or other means of abrasion in order to promote good adhesion consistently. Sanding is a relatively time consuming step in assembly line operations. The added time required to properly prepare substrate surfaces for bonding disadvantageously results in significantly higher manufacturing costs.
A process for bonding SMC substrates by means of a temperature resistant bonding system, which eliminated the need to abrade substrate surfaces in order to achieve good bonding, which thus be a useful addition to the SMC molding art and other arts requiring high temperature resistance of adhesively bonded polymer layer.